Daylight Saving Time: Why Some Days Have 23 or 25 Hours

What actually changes

A place on daylight saving time changes its UTC offset twice a year. In the US, the clocks in most areas go forward at 02:00 local time on the second Sunday in March, jumping from 01:59 to 03:00, and back at 02:00 on the first Sunday in November, when 01:59 is followed by 01:00 again.

• Spring change: the clock skips an hour. Local times from 02:00 to 02:59 never happen that day, and the day has 23 hours.
• Autumn change: the clock repeats an hour. Local times from 01:00 to 01:59 happen twice, and the day has 25 hours.
• In 2026 the US changes happen on Sunday 8 March and Sunday 1 November.

Other regions use other rules. The European Union changes on the last Sunday in March and the last Sunday in October, at the same instant in every member state, and the UK uses the same dates. Places in the southern hemisphere that observe it have their summer in the opposite half of the year, so their changes fall the other way round. Many countries, including India and Japan, do not change their clocks at all. Rules are set by law and can be altered, so confirm the current ones for the dates you need.

Worked example: the 22:00 to 06:00 shift

On an ordinary night, 22:00 to 06:00 is 8 hours. Take a worker in New York on the nights the clocks change.

• Night of 7 to 8 March 2026 (spring forward): 22:00 EST is 03:00 UTC, and 06:00 EDT the next morning is 10:00 UTC. The elapsed time is 10 − 3 = 7 hours.
• Night of 31 October to 1 November 2026 (fall back): 22:00 EDT is 02:00 UTC, and 06:00 EST is 11:00 UTC. The elapsed time is 11 − 2 = 9 hours.

Subtracting the two clock readings gives 8 hours both times, which is wrong for the real elapsed time. How such a shift should be paid or recorded depends on the employer's policy and local rules, but the arithmetic needs to start from the true duration.

When two regions change on different dates

The US and the UK change their clocks on different dates, so the time difference between New York and London is not constant. It is normally 5 hours, but it shrinks to 4 hours for a short time in spring and again in autumn.

The effect: a weekly call fixed at 10:00 New York time is 15:00 in London for most of the year, but 14:00 in London during the four-hour weeks. A call fixed in London time shifts by an hour for New York instead. Neither party has done anything wrong; the offsets simply moved on different dates.

Practical rules for safe calculations

• Measure durations in UTC. Convert both ends to UTC, then subtract. The result is correct whatever the local clocks did in between.
• Count days by calendar date. The number of days between two dates is unaffected by DST. Only hour-and-minute spans are.
• Decide what a recurring event keeps fixed. The local time (a 09:00 stand-up) or the UTC time? State it, and say whose local time.
• Avoid the changeover window. A 02:30 appointment on the spring-forward date does not exist, and on the autumn date it is ambiguous.
• Use zone names, not fixed offsets. Store a named region such as New York rather than "UTC−5", so the correct offset applies on each date.
• Check the dates every year. They are moving dates, such as the second Sunday of March, not fixed ones.

A time zone converter picks the right offset for any date. The hours between times calculator and the date and time duration calculator are useful for checking a duration, as long as you enter times that share a zone.